What are they for? I'm talking about heads that have been obviously designed with a "pocket" formed of ribs, the valve cover rail and a valve guide boss. This pocket traps oil from the cam bearings and holds it next to the valve guide boss. This oil does not drain unless the head is inverted. I've only seen them in overhead cam engines, and they seem to be getting more popular with manufacturers.
I suspect these are used to cool the valve guides, but it seems to me a constant flow of oil would cool far more effectively than the simple heat-sink these "pockets" provide. Also, this is dirty oil that remains in the engine during an oil change.
On my 1994 Toyota V6 pickup, I find myself tempted to carefully drill a 0.020-0.040" hole in the bottom or side of these things to allow the oil to drain to the crankcase at a controlled rate. That is enough to drain the "pocket" when the engine is stopped, and also allow the pocket to fill & overflow during operation. This would also provide more oil "flow-through" during operation, resulting in more cooling of the guide.
Well-thought responses invited.